Why Airplane Cabin Windows Have Holes

The cabin windows on a typical passenger plane actually have not one, not two, but three window panes (I’ll refer to them as the outer, middle, and inner panes). The outer and middle panes are virtually identical in construction and are integrated into a single frame with a combined seal, as you can see in the following photo and schematic…
The middle pane mainly serves as a reserve window, designed to back up the outer pane in the unusual event that it gets damaged mid-flight. In normal operation, the outer window is the one which bears the full load of the pressure differential between the pressurized cabin and the low atmospheric pressure outside. The reason for the small ‘bleed hole’ or ‘breather’ in the middle pane is to equalize the pressure on both of its faces so that it doesn’t bear any of this differential load. If the outer pane did happen to shatter, the middle pane would then bear the full load (albeit with a small loss through the hole which the cabin pressurizing system could easily compensate for). Here’s the friendly GIF summary…

As for the inner pane, it simply serves as a transparent ‘scratch pane’ or ‘dust cover’ protecting the inner assembly. Note that the scratch pane is not perfectly sealed, but rather allows air to flow, and hence pressure to equalize, between the cabin and the structural window assembly.

An additional benefit of this design is that the inner pane is insulated from the often freezing external temperature, ensuring that it doesn’t fog up due to the condensation of water vapor inside the cabin. Clever huh?